A variety of magnetic sensors are used for detecting or measuring magnetic fields, such as superconducting quantum interference device (SQUID) fluxmeters capable of measuring extremely weak magnetic fields of 10.sup.-14 T (tesla) or semiconductor Hall element magnetic sensors that are able to measure ultra-strong magnetic fields of 10 T or more. Such magnetic sensors are widely used, not only for measuring the intensity of magnetic fields but also for detecting currents or, in combination with magnetic substances, for measuring other physical quantities such as position and acceleration or for switching as switching elements. FIG. 7 shows operable ranges and major application fields of various well-known magnetic sensors (according to Kiyoshi Takahashi et al., "Dictionary of Sensors", Asakura shoten, 1991).
As shown in FIG. 7, conventional measurement of magnetism has used various magnetic sensors having different operational principles and structures in a wide ranges of measurement, which has resulted in the need to properly use them depending on their measuring ranges. Further, the above described variety of sensors must be used under limitations in their application conditions. For example, the sensor of a SQUID fluxmeter must be cooled to a very low temperature and thus it cannot be readily used in many applications.
Semiconductor magnetic sensors are widely used among conventional magnetic sensors because they are capable of measuring magnetic fields in a relatively wide range. As to the operational principles of semiconductor magnetic sensors, the Hall effect is most commonly applied and utilized as Hall element magnetic sensors.
The Hall effect will now be briefly described. When a magnetic field (with a magnetic flux density B) is applied in a direction perpendicularly to the direction of a current (I) flowing through a conductor, a Hall electrical field is produced in a direction perpendicular to both of the directions of the current and applied magnetic field to generate a voltage across a conductor. This is referred to as a Hall voltage V which is expressed by: EQU V.sub.H =K.sub.H .multidot.B.multidot.I (1)
where d, w, and L respectively represent the thickness, width, and length of the conductor through which the current flows. That is, the magnetic flux density B can be identified by measuring the Hall voltage V.sub.H which is proportional to the magnetic flux density B. K.sub.H represents product sensitivity which is expressed by the following equation. EQU K.sub.H =R.sub.H .multidot.f.sub.H /d (2)
where R.sub.H represents a Hall coefficient, and f.sub.H represents a shape factor which is a value related to L/w and a Hall angle .theta. and therefore approaches 1 with an increase in .theta., i.e., with increases in Hall mobility .mu..sub.H =R.sub.H .multidot..sigma. (.sigma.: electrical conductivity) and the magnetic flux density. Therefore, from Equations (1) and (2) we obtain: EQU V.sub.H =R.sub.H .multidot.f.sub.H .multidot.I/d. (3)
If we assume a driving under a constant voltage, Equation (3) gives: EQU V.sub.H =(w/L)R.sub.H .multidot.f.sub.H .multidot..sigma..multidot.B.multidot.V (4)
where V is the applied voltage.
Therefore, the Hall voltage V.sub.H can be effectively increased by increasing (w/L), R.sub.H, and .sigma.. However, since (w/L) is a parameter which depends on the shape of the device and no significant change occurs in f.sub.H when the Hall angle .theta. is large, R.sub.H and .sigma. become the key factors.
(a) R.sub.H is given by the following equation where n represents an electron density; e represents the charge of electrons; and c represents speed of light. EQU R.sub.H =-(nec).sup.-1 (CGS system) (5) PA1 That is, R.sub.H is inversely proportional to the electron density n. PA1 (b) .sigma. is given by the following equation where .tau. represents the time between electrons collisions and m* represents the effective mass of electrons. EQU .sigma.=ne.sup.2 .tau./m* (6) PA1 That is, .sigma.is proportional to n and .tau.. As is apparent from Equations (5) and (6), the electron density n affects R.sub.H and .sigma. oppositely. Therefore, it is not an effective factor for increasing both R.sub.H and .sigma.. PA1 (1) To improve magnetic sensitivity and expand the measuring range, i.e. to improve sensitivity and to allow a single magnetic sensor to cover a wide measuring range. PA1 (2) To expand application conditions--to give excellent environmental range--proof characteristics and especially to enable operations at high temperatures.
A magnetic sensor comprising a semiconductor Hall element is compact and capable of measuring a magnitude of magnetic field of in a relatively wide range as shown in FIG. 7. However, its sensitivity and resolution are not satisfactorily suited for measuring magnetic flux densities of 10.sup.-9 T or less. Further, a semiconductor can not be used at high temperatures which causes the semiconductor to rapidly increase its carrier density. In addition, such a device has a problem in that it is susceptible to radiations. Thus, the fields of application of such a device is actually limited due to limitations in application conditions.
It is an object of the present invention to solve the following two problems.